Dark material on Vesta from the infall of carbonaceous volatile-rich material

• Published in: Nature (London) Vol. 491; no. 7422; pp. 83 - 86
• Main Authors: McCord, T. B., Li, J.-Y., Combe, J.-P., McSween, H. Y., Jaumann, R., Reddy, V., Tosi, F., Williams, D. A., Blewett, D. T., Turrini, D., Palomba, E., Pieters, C. M., De Sanctis, M. C., Ammannito, E., Capria, M. T., Le Corre, L., Longobardo, A., Nathues, A., Mittlefehldt, D. W., Schröder, S. E., Hiesinger, H., Beck, A. W., Capaccioni, F., Carsenty, U., Keller, H. U., Denevi, B. W., Sunshine, J. M., Raymond, C. A., Russell, C. T.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.11.2012; Nature Publishing Group
• Subjects: 639/33/445; Albedo; Asteroids; Asteroids (minor planets); Astronomy; Carbonaceous rocks; Chemical composition; Earth, ocean, space; Exact sciences and technology; Gamma rays; Humanities and Social Sciences; Hypotheses; letter; Meteors & meteorites; Moons; multidisciplinary; Natural history; Planetary, asteroid, and satellite characteristics and properties; Planets, their satellites and rings. Asteroids; Properties; Pyroxene; Saturn; Science; Solar system; Spectra; Vesta (Asteroid); United States; Comets; Mixing; Albedo; Meteorites; Asteroids; Surface properties; Regolith
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature11561

From spectra and images of Vesta, it is suggested that the dark patches on Vesta are formed of infalling hydrated carbonaceous material and the bright patches are uncontaminated Vesta soil. A Dawn view of Vesta Between 16 July 2011 and 5 September 2012, NASA's space probe Dawn was orbiting Vesta, a protoplanet thought to have survived virtually intact since an early phase of Solar System formation. In this issue of Nature , two groups report on the encounter. Carle Pieters and co-workers find that space weathering on Vesta has followed a different course from that observed on the Moon and on Itokawa, the asteroid sampled in an Earth-return mission. On Vesta, weathering involved fine-scale regolith (soil) mixing that has removed clear traces of recent impact deposits. There are no signs of the nanophase metallic-particle deposits seen on the Moon and Itokawa. Thomas McCord and co-authors describe two main types of material on Vesta's surface: bright and dark. The bright material may be uncontaminated indigenous Vesta basaltic soil, with the darker material derived from low-albedo impactors. Dawn has now moved on and is due to rendezvous with the protoplanet Ceres in February 2015. Localized dark and bright materials, often with extremely different albedos, were recently found on Vesta’s surface 1 , 2 . The range of albedos is among the largest observed on Solar System rocky bodies. These dark materials, often associated with craters, appear in ejecta and crater walls, and their pyroxene absorption strengths are correlated with material brightness. It was tentatively suggested that the dark material on Vesta could be either exogenic, from carbon-rich, low-velocity impactors, or endogenic, from freshly exposed mafic material or impact melt, created or exposed by impacts. Here we report Vesta spectra and images and use them to derive and interpret the properties of the ‘pure’ dark and bright materials. We argue that the dark material is mainly from infall of hydrated carbonaceous material (like that found in a major class of meteorites and some comet surfaces 3 , 4 , 5 ), whereas the bright material is the uncontaminated indigenous Vesta basaltic soil. Dark material from low-albedo impactors is diffused over time through the Vestan regolith by impact mixing, creating broader, diffuse darker regions and finally Vesta’s background surface material. This is consistent with howardite–eucrite–diogenite meteorites coming from Vesta.